1. Field of the Invention
The present invention relates to a design assisting method, a design assisting system, and a design assisting tool for use in designing printed-circuit boards or other electronic devices, and more particularly to a design assisting method and a design assisting system for designing electronic devices with reduced electromagnetic radiations.
2. Description of the Prior Art
It is necessary to design printed-circuit boards and electronic devices with reduced electromagnetic radiations in order to prevent any unwanted electromagnetic radiations from those printed-circuit boards and electronic devices from interfering the reception of broadcasts and communications and also from causing other electronic devices from malfunctioning. However, since it has been customary to design printed-circuit boards and electronic devices based on the experiences and know-hows of circuit designers, it has been difficult for anybody to design products incorporating effective countermeasures against unwanted electromagnetic radiations. In order to solve such a problem, there have been proposed various tools for assisting in designing printed-circuit boards and electronic devices with reduced electromagnetic radiations. For example, the proposals include Japanese laid-open patent application No. 10-049568 (JP, 10049568, A) and Japanese laid-open patent application No. 10-091663 (JP, 091663, A).
FIG. 1 is a flowchart illustrating the method for generating the layout of a printed-circuit board disclosed in JP, 10-049568, A. The disclosed method is based on the idea that the dominant radiation from a printed-circuit board originates from signal lines, and characterized by calculating the amount of radiation from signal lines, providing a countermeasure if the calculated amount of radiation exceeds a certain limit value, and determining an optimum layout for the countermeasure. The disclosed method allows a printed-circuit board to be designed with reduced electromagnetic radiations from the signal lines.
In step S1 shown in FIG. 1, parts layout information indicative of the layout of parts and connection base information indicative of connections between the parts are determined using a conventional CAD (computer-aided design) system or a conventional circuit simulator. In step S2, board data as a basis for determining line constants for signal lines, such as conductor thicknesses, at the time circuits are formed on an actual board, and signal characteristics indicative of the characteristics of signals applied to the signal lines, such as device models representing input and output characteristics of the parts, are entered. A number 1 is set as an initial value for a table number m that is used for referring to a layer structure description table, and a number 1 is specified as an initial value for a cross-section number n in a hypothetical cross-section description table.
In step S3, hypothetical interconnection paths between the parts are calculated from the parts layout information and the connection base information both have been determined in step S1. In step S4, an amount X of unwanted radiation is calculated for each of the determined hypothetical interconnection paths. If the amount X of unwanted radiation is in excess of a preset allowable value A, then, in step S5, noise countermeasures are taken to reduce the amount X to or below the preset allowable value A. The noise countermeasures include two noise countermeasure procedures, i.e., a procedure of improving a hypothetical cross-section to strengthen the ground, and another procedure of inserting a noise countermeasure component such as a capacitor. In step S6, it is determined whether there is a signal line yet to be processed or not. If there is a signal line yet to be processed, then control goes back to step S3 in order to repeat the processing in steps S3 through S5 for that signal line.
In step S7, with respect to all the signal lines whose amount X of unwanted radiation has exceeded the preset allowable value A, improved solution N1 is calculated from the interconnection information with an improved cross-sectional shape, and improved solution N2 is calculated from the interconnection information with an inserted noise countermeasure component. If improved solutions N1, N2 are not particularly distinguished from each other, then they are simply referred to as improved solutions N.
After improved solutions N are determined in step S7, a description table and a described layer structure are assigned to each of improved solutions N. In step S8, it is then reviewed whether each of the layer structures can be practically feasible or not. In step S9, practically feasible solutions P are extracted from the combination of the layer structures and improved solutions N. Specifically, interconnection complexity levels xcex1 and variations xcex3 of interconnection complexity levels xcex1 are calculated with respect to the respective layer structures, and it is determined whether each of the layer structures is practically feasible or not based on whether or not interconnection complexity level xcex1 and variation xcex3 thereof are equal to or smaller corresponding allowable values B, C. In step S10, optimum solution Q is selected from the collection of calculated practically feasible solutions P by evaluating sum x of amounts X of unwanted radiation and manufacturing cost y. Thereafter, in step S11, signal lines are actually placed on a layer structure determined by selected optimum solution Q. In this manner, interconnections on a printed-wiring board are determined.
FIG. 2 shows a conceptual presentation of operation of the CAD apparatus revealed in JP, 10091663, A. The disclosed CAD apparatus resides in that when a certain interconnection is specified on the CAD apparatus used for designing a printed-circuit board, an amount of radiation from the interconnection is calculated based on signal waveform information of the interconnection, and the intensity of radiation from place to place is visually displayed. The CAD apparatus is capable of identifying the position of a dominant signal interconnection which produces electromagnetic radiation, and hence permits a countermeasure to be easily taken against the radiation from the signal interconnection. The disclosure is characterized in that basic period T, voltage amplitude V0, rise and fall times tr, logic high period t0, and duty ratio xcfx84 (=(trxe2x88x92t0)/T) of the signal waveform can be described, and the printed-circuit board CAD apparatus calculates a current based on these descriptions. In FIG. 2, frames A1, A2, A3, A4, . . . schematically represent the concepts of these quantities and quantities derived therefrom. A trapezoidal signal shown in frame A1 comprises a plurality of harmonics. When the circuit designer observes an n-th harmonic, if observed frequency f is f=n/T as indicated in frame A2, current I(f) can be calculated according to the equations in frames A3, A4.
If it is assumed that the interconnection layer is made of a metal foil having width xcfx89, has interconnection length L, and one mesh used in calculations carried out by the CAD apparatus has area A, then a current density per mesh is expressed by I(f)xc2x7xcfx89xc2x7L/A. The current density per mesh is calculated for each mesh, and each mesh is grouped into a level depending on the calculated current density, and displayed on the display screen of the CAD apparatus. As indicated by examples in frame A6, the levels are displayed in four or more luminance gradations, colors, or patterns.
In this manner, the CAD apparatus shown in FIG. 2 appropriately displays the concentration of radiation noise in each area, allowing interconnections to be designed according to an interactive editing process.
Unwanted electromagnetic radiation will be described below.
In the technical field of unwanted electromagnetic radiation, high-frequency currents (or radio-frequency currents) are roughly divided into xe2x80x9cdifferential-mode currentxe2x80x9d and xe2x80x9ccommon-mode currentxe2x80x9d. The xe2x80x9cdifferential-mode currentxe2x80x9d refers to currents of the same magnitude flowing in opposite directions in a signal interconnection and a ground plane that faces the signal interconnection, and are also called a xe2x80x9cloop currentxe2x80x9d. The xe2x80x9ccommon-mode currentxe2x80x9d refers to a differential current that is generated when a signal interconnection current and a ground plane current are brought out of balance for some reason. A current flowing through a conductor such as an element of a dipole or monopole antenna, i.e., a current that lacks a paired current in the vicinity, is also referred to as a common-mode current.
If the differential-mode current has a small value, then its radiation level causes no problem because currents of the same magnitude and in opposite phase flow in the vicinity of each other and hence electromagnetic fields generated thereby cancel each other. However, the common-mode current brings about a strong radiation even if its value is small because there is no canceling paired current in the vicinity. The differential-mode current can easily be recognized as a current flowing through a signal interconnection. However, it has been impossible so far to recognize where and how the common-mode current flows and to find its value because the cause of the common-mode current is not known. Stated otherwise, the differential-mode radiation can easily be suppressed by the product design and layout, whereas the common-mode radiation is difficult to reduce. Usually, the overall radiation ability of products is considered to be determined by the common-mode radiation.
The techniques disclosed in JP, 10049568, A and JP 10091663, A are primarily aimed at the suppression of a radiation from signal interconnections on printed-circuit boards, and are each concerned with a tool for assisting in designing an electronic device and a printed-circuit board with a suppressed radiation due to a differential-mode current, and a board structure. The disclosed techniques are not directed to the suppression of a radiation due to a common-mode current.
FIG. 3 schematically shows relation between the layout of a printed-circuit board and the flows of high-frequency currents that cause radiations. As shown in FIG. 3, a ground plane and two LSI (large-scale integration) circuits are mounted on a printed-circuit board. A differential-mode current is indicated by the broken-line arrow, and a common-mode current is indicated by the solid-line arrow. The inventions disclosed in the above publications are addressed to a radiation from the differential-mode current that flows through the signal interconnection and the ground plane which are paired.
Our current study has indicated that radiations from electronic devices which incorporate printed-circuit boards are often mainly composed of a radiation produced when the ground plane on the board acts as an antenna and a radiation when a cable connected to the ground plane acts as an antenna, rather than a radiation caused by the differential-mode current. Those radiations are produced by the common-mode current. The conventional procedures and apparatus have been unable to assist in designing electronic devices and printed-circuit boards with a suppressed radiation due to a common-mode current because the behavior of the common-mode current cannot be recognized.
It is an object of the present invention to provide a design assisting system for designing a printed-circuit board with a reduced common-mode current which would be responsible for an increased electromagnetic radiation, a printed-circuit board to which a cable is connected, and an electronic device.
Another object of the present invention to provide a design assisting method for designing a printed-circuit board with a reduced common-mode current which would be responsible for an increased electromagnetic radiation, a printed-circuit board to which a cable is connected, and an electronic device.
According to an aspect of the present invention, there is provided a design assisting system comprising means for converting at least a set of an electronic device, an interconnection, and a ground plane from layout information of a circuit board into a model for analyzing an electromagnetic field, means for specifying a frequency and calculating a magnetic field intensity distribution near the ground plane using the model, and means for superposing the calculated magnetic field intensity distribution and the position of the interconnection, determining whether a position where a magnetic field or a current is strong and the position of the interconnection are close to each other or not, and outputting a determined result.
According to another aspect of the present invention, there is provided a design assisting system comprising means for converting at least a set of an electronic device, an interconnection, and a ground plane from layout information of a circuit board into a model for analyzing an electromagnetic field, means for entering a an assumed position where a cable is connected, means for specifying a frequency and calculating an electric field intensity distribution near the ground plane using the model, and means for determining whether a position where an electric field is strong and the assumed position are in agreement with each other or not, and outputting a determined result.
According to still another aspect of the present invention, there is provided a design assisting system comprising means for converting at least a set of an electronic device, an interconnection, and a ground plane from layout information of a circuit board into a model for analyzing an electromagnetic field, means for specifying a frequency and calculating an electric field intensity distribution near the ground plane using the model, and means for finding a position whether an electric field is weak as a position suitable for cable connection, and outputting the position.
According to yet another aspect of the present invention, there is provided a design assisting method comprising the steps of converting at least a set of an electronic device, an interconnection, and a ground plane from layout information of a circuit board into a model for analyzing an electromagnetic field, specifying a frequency, calculating a magnetic field intensity distribution near the ground plane using the model at the specified frequency, superposing the calculated magnetic field intensity distribution and the position of the interconnection thereby to determine whether a position where a magnetic filed or a current is strong and the position of the interconnection are close to each other or not, and outputting a determined result.
According to yet still another aspect of the present invention, there is provided a design assisting method comprising the steps of converting at least a set of an electronic device, an interconnection, and a ground plane from layout information of a circuit board into a model for analyzing an electromagnetic field, entering an assumed position where a cable is connected, specifying a frequency, calculating an electric field intensity distribution near the ground plane using the model at the specified frequency, determining whether a position where an electric field is strong and the assumed position are in agreement with each other or not, and outputting a determined result.
According to a further aspect of the present invention, there is provided a design assisting method comprising the steps of converting at least a set of an electronic device, an interconnection, and a ground plane from layout information of a circuit board into a model for analyzing an electromagnetic field, specifying a frequency, calculating an electric field intensity distribution near the ground plane using the model at the specified frequency, and finding a position whether an electric field is weak as a position suitable for cable connection based on the electric field intensity distribution.
The present invention also provides a computer-readable medium storing a program enabling a computer to perform each process of the above design assisting methods.
With the above arrangement, there is realized a design assisting system capable of suppressing a radiation due to a common-mode current. Since a factor, i.e., a magnetic field intensity distribution or an electric field intensity distribution in the vicinity of a ground plane, for determining a radiation due to a common-mode current is clarified, it is possible to assist designs and modifications of printed circuit boards or electronic devices once points to be checked are clarified according to the present invention. Furthermore, because minimum elements required by an electromagnetic field analyzing model for finding the factor for determining a radiation due to a common-mode current, and procedures for laying out those elements are also clarified, it is possible to obtain information required by designs in a short period of time according to the present invention. For designing a printed-circuit board, it may be sufficiently advantageous in certain situations to obtain information that is 50% accurate in one minute, rather than to obtain information that is 90% accurate in one week. The design assisting system and method according to the present invention are effective as a design assisting tool in such an application.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.